Pitting Behavior of S2205 Stainless Steel in NaClO₂ Solution at Initial Stage

Through microstructure analysis, composition analysis, and corrosion morphology observation of the failed S2205 stainless steel pipe in NaClO₂ solution, it was clear that severe pitting corrosion was the main reason for the failure of the pipe. The electrochemical slow dynamic potential scanning method was used to analyze the metastable pitting potential E_m, the steady-state pitting potential E_b, the number of nucleation of the metastable pitting and the pore growth rate and other parameters. The initial behavior of pitting of S2205 stainless steel was studied. The results showed that at the initial stage of metastable pitting of S2205 stainless steel, E_m was less affected by the concentration of NaClO₂, E_b decreased with the increase of NaClO₂ concentration, and the pitting sensitivity increased. Metastable pitting nucleation was dominated by the induction of metal surface inclusions, and had little relationship with the change of NaClO₂ concentration. In the higher potential range, the metal inclusions dissolved to less than the critical size and could not affect the nucleation of metastable pitting. The high potential inhibitted the nucleation of metastable pitting corrosion. As the concentration of NaClO₂ increased, Cl^- increased and accelerated its diffusion into the pores and promoted the growth of metastable pitting.The higher potential increased the activity of Cl^- and promoted the growth of metastable pitting, and the higher the concentration, the more obvious the promotion effect.